A microspeaker is provided in a portable device, etc. to generate the sound. With recent developments of mobile devices, the microspeaker has been mounted in various mobile devices. In particular, the latest mobile devices tend to have a light weight, small size, and slim shape to facilitate portability, and accordingly, the microspeaker mounted in the mobile devices is required to have a small size and slim shape.
However, for a microspeaker having a small size and slim shape, an area of a diaphragm decreases, and a volume of a resonance space, in which the sound generated by the vibration of the diaphragm is resonated and amplified, also decreases, as a result of which a sound pressure decreases. Such decrease in the sound pressure is particularly pronounced at low frequencies. According to the prior art, in order to enhance a low-frequency sound pressure, air adsorbents which are porous materials are arranged in a resonance space, such that the porous materials adsorb air molecules to define a virtual acoustic space, which results in increased low-frequency SPL and decreased low-frequency THD. Examples of efforts in this regard include EP 2,424,270 B1 and U.S. Pat. No. 8,687,836 B2.
As illustrated in FIG. 1, the applicant has also developed a technology of filling air adsorbents in an enclosure with a microspeaker therein so as to increase low-frequency SPL. A microspeaker A is mounted in an enclosure, a predetermined amount of porous materials n are filled in a resonance space (back volume) in the enclosure, and the air occupies the remaining space. The enclosure includes an upper casing 11, a lower casing 12, a filling hole 13 which is formed in the lower casing 12 and through which the porous materials are to be filled in the enclosure, and a cover 14 for covering the filling hole 13. A support member 21 and 22 may be provided in the upper casing 11 or the lower casing 12, that serve to guide the microspeaker A to be properly mounted and also serve to separate the microspeaker A from the lower casing 12 so that the air can smoothly flow in and out via a vent hole formed in a yoke of the microspeaker A. The support members 21 and 22 can be integrally formed with the upper casing 11 and the lower casing 12, respectively.
However, as can be seen in FIG. 1, if the porous materials n are not filled in the resonance space up to 100% during the filling of the porous materials, the porous materials n may collide with one another and with the enclosure 11, 12 and 14 or the microspeaker A, causing noise. Such noise definitely results from the vibration of the porous materials n, since it is not generated without the movement of the porous materials. Therefore, in the case that the porous materials are filled in the resonance space, if the microspeaker generates the sound, it can be seen that such sound is caused by the collision between the inner components of the enclosure and the porous materials. Thus, there is the need to remove noise caused by the vibration of the porous materials, while increasing low-frequency SPL and decreasing low-frequency THD by the porous materials.